Signal Variables Shape Wireless

July 2, 2008
Infrastructure Hardware and Transmission Frequency Depend Largely on Transmission Speed, Amount of Data, Distance and Obstructions

By Don Talend

New wireless infrastructure hardware and security protocols give managers unprecedented monitoring and control of processes in challenging situations. A company’s choices in wireless infrastructure hardware and transmission frequency depend greatly on the necessary transmission speed, the amount of data transmitted, distance and signal obstructions. Regardless of the infrastructure configuration, more-effective security recently has become available to safeguard it.

“When you’re looking at wireless, the lower the frequency, the better it’s going to penetrate obstacles,” says Ira Sharp, lead product marketing specialist, interface—wireless, for Phoenix Contact, “So if you have a long-distance application that has to go through walls, foliage or trees, you want as low a frequency as possible.

High frequencies are more affected by those obstacles, and your signal won’t go as far. However, the lower frequencies, 400 and 900 MHz, aren’t going to give you the bandwidth to communicate large amounts of data. Higher-frequency 2.4 and 5-GHz hardware is going to provide faster communication and can communicate more data.”

Sharp details the major components in a wireless network. A host radio, a.k.a. master or access point, connects the user’s wireless network to its wired network. A node or remote radio/slave is located at each monitoring or control point. He says Phoenix Contact offers a two-way radio transceiver set with corresponding digital signal I/Os. Discrete inputs might come from a programmable logic controller card, and the transceiver transmits outputs to a slave or node field device, he says, adding that the company has repeaters—radios that are useful for transmitting signals between the node and master in cases with obstructions or where distances are such that the signal needs repeating.

Bruce Hofmann, director of marketing for Weidmüller North America, says when a company’s monitoring and controlling needs grow, a transition from I/O to wireless gateways and wireless modems makes sense.
“We don’t recommend radio I/O for machine control,” warns Hofmann, adding that typical delays of a few msec inherent in I/O can disrupt a fully automated process and that I/O is better suited to transmitting data at a particular time of day or to report a change of state.

When the customer crosses a particular threshold of points to monitor or control, a wireless gateway is an option, says Hofmann. “The problem we’ve found is that most I/O configurations aren’t scalable from hundreds of points to a PLC. Wireless gateways in our product line are transceivers but also have a built-in register table and are coded to whatever PLC you want to control. Six different gateway versions are available for compatibility with multiple manufacturers’ PLCs and have 4,000 registers for monitoring 4,000 points in one PLC.”

He also points out that the most robust wireless network is tied to a fully automated production process and wireless modems are better suited to this application due to their speed of signal transmission. Compared with a 2.4-GHz router with which a computer user would wirelessly access the Internet at home at 20 mW of power, he says, an industrial wireless modem will send the signals at about 15 times the power.

“The 2.4-GHz router is like a big, fat pipe; you can move a lot of information,” says Hofmann. “It’s like a shotgun, but it won’t shoot through a wall very well. Our wireless modems send signals 3 miles instead of 300 ft. In factory automation, you don’t want to go 3 miles, but you can go through most obstructions.”

Hofmann says a 900-MHz wireless modem often is well-suited to a factory-automation application because it’s more like a rifle bullet than a shotgun blast. “People in factory automation want to move, say, 115 kilobits and blow through concrete walls,” he says. “The 900-MHz wireless modem gets through electrical interference.”

Wireless infrastructures serve functions ranging from simple monitoring to more sophisticated command-and-control functions, says Sharp. “If you have a facility where you need to communicate over long distances, you don’t need really great speed because you’re typically turning on and off a pump or monitoring a level.”

That type of wireless application is one in which managers are just getting their feet wet, adds Sharp. “Some of the more-advanced wireless uses more of our high-frequency equipment to provide a wireless Ethernet connection to an end device,” he says. “We’ve done a lot of work with cardboard manufacturers, automotive manufacturers—anybody who has an automatic guided vehicle or cart. More often than not, the control would be done by a PLC on the device, and you give commands to the PLC via the wireless device.”

In the software arena, Keith Jones, marketing manager for Wonderware, says customers have realized more robust process-monitoring capabilities in recent years. The company’s InTouch HMI software has added functionality for wireless applications.

Compatibility with operating-system extensions, such as Microsoft Windows Tablet PC, is built into the software. “Using handheld industrial tablet PCs, many customers roam around their facilities and remain in contact with their processes,” says Jones. “The software can make screen shots of process anomalies, and users can mark up the screen shots and email them for action. The user also can access plant information via an intranet using PDA devices.”

As the use of wireless control increased in recent years, security improved. Sharp notes that proprietary or public standard RF systems are available. “A proprietary system is designed and maintained by a particular manufacturer,” he says. “You can’t just get onto it with a standard radio. You’re married to a particular manufacturer.” A public standard RF system based on a platform such as Bluetooth is more flexible and allows network components to communicate, but until recently, they have been less secure, explains Sharp.

“There has been a huge advancement in wireless security for proprietary systems,” says Sharp. The first wireless security specification, Wired Equivalent Privacy, was hacked immediately, he says, but Wi-Fi Protected Access (WPA) is more difficult to hack but is not entirely reliable. The latest is WPA-2, which uses Advanced Encryption Standard—an encryption standard adopted by the U.S. government to protect classified information. “Just recently, security advancements have affected wireless hardware,” says Sharp. WPA-2, because it has so much data on it, needs a co-processor, or a processor capable of handling that type of encryption, so there has been an architecture change.”

For added security, Sharp recommends ensuring that a wireless network only sends signals as far as the application requires. “There are two types of antennas: directional and omni-type, which allows you to communicate 360° around the antenna,” he says. “I recommend that remote sites use a directional antenna so you can focus the signal directly back to the master.”

ProSoft Technology, a provider of connectivity and communication hardware, provides an example to illustrate the reliability and security challenges that can affect a monitoring application adversely. Gainsville, Texas, is in Cooke County, which derives 82% of its income from cattle, making it an environmentally sensitive area for oil companies. The Walnut Bend oil field in Gainsville, managed by Gruy Petroleum Management, produces about 750 barrels of oil per day from 104 wells. The oil field’s 150-MHz licensed-frequency radio monitoring system recently was experiencing significant power grid interference, which caused the system to overload and shut down spontaneously.

Automation Alternatives in Weatherford, Texas, provided a solution that facilitates immediate system shutdown when an alarm indicates a spill. Schneider Electric and Allen-Bradley PLCs receive analog and discrete data from the field and wirelessly transmit it using 22 ProSoft Technology 2.4-GHz serial radios. A National Instruments Lookout SCADA system polls facilities every five minutes for variables such as tank levels, line pressures and discrete alarms. The new radios allow central field monitoring 24/7 and are free of interference for a secure solution.

Don Talend of Write Results, West Dundee, Ill., is a communications and publicity consultant specializing in the technical trade media. Email him at [email protected].

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